feat : pacific-atlantic-water-flow

Author: ekgns33

pacific-atlantic-water-flow/ekgns33.java

@@ -0,0 +1,62 @@
+/*
+* input : grid represents the heights of island
+* output : coordinates where water flows to both pacific and atlantic
+*
+* example
+*
+*  2 2 2   1,1 >> 1,2 >> atlantic / 1,1 >> 0,1 >> pacific
+*  3 3 1
+*  4 3 1
+*
+* do dfs/bfs from the edge of grids.
+* if cell is checked from pacific and atlantic add to result
+* solution  dfs from edges
+*   tc : O(m*n)
+*   sc : O(mn)
+*
+* */
+class Solution {
+  public List<List<Integer>> pacificAtlantic(int[][] heights) {
+    int m = heights.length;
+    int n = heights[0].length;
+    int[][] pacific = new int[m][n];
+    int[][] atlantic = new int[m][n];
+
+    for(int i = 0; i < m; i++) {
+      dfsHelper(heights, pacific, i, 0);
+    }
+    for(int j = 1; j < n; j++) {
+      dfsHelper(heights,pacific, 0, j);
+    }
+    for(int i =0; i < m; i++) {
+      dfsHelper(heights,atlantic, i, n-1);
+    }
+    for(int j = 0; j < n-1; j++) {
+      dfsHelper(heights, atlantic, m-1, j);
+    }
+    List<List<Integer>> ans = new ArrayList<>();
+    for(int i = 0; i < m; i++) {
+      for(int j = 0; j < n; j++) {
+        if(pacific[i][j] == 1 && atlantic[i][j] == 1) {
+          ans.add(List.of(i, j));
+        }
+      }
+    }
+    return ans;
+  }
+
+  static int[][] directions = {
+      {1, 0}, {-1, 0}, {0, 1}, {0, -1}
+  };
+
+  void dfsHelper(int[][] board, int[][] visited, int x, int y) {
+    if(visited[x][y] > 0) return;
+    visited[x][y] += 1;
+    for(int[] direction : directions) {
+      int nx = x + direction[0];
+      int ny = y + direction[1];
+      if(nx < 0 || nx >=visited.length || ny < 0 || ny >= visited[0].length || visited[nx][ny] > 0 || board[nx][ny] < board[x][y]) continue;
+      dfsHelper(board, visited, nx, ny);
+    }
+  }
+}